Non-Aromatic Heterocycles with sp2 Ring Carbons

Carbon-13 shift of common non-aromatic heterocycles with endo- and exocyclic double bonds are reviewed in Table 4.66 [416-432], - Deshieldings of / -carbons induced by carbonyl groups in heterocyclic a, /1-enones due to (—)-M electron withdrawal (e.g. 2-pyrones, coumarins) and shieldings of [ carbons in cyclic enol ethers arising from (+ )-M electron release (e.g. 2,3-dihydrofuran and oxepine derivatives in Table 4.66) fully correspond to the effects described for the open-chain analogs. Outstandingly large shift values are observed for the lithiated carbon in cyclic a-lithium enol ethers (Table 4.66). In terms of its a and / carbon-13 shifts, 2,7-dimethyloxepine is also a typical enol ether [420], Further, 2,6-dimethyl-4-pyrone [421] and flavone [422] display similiar shift values for the a, /1-enone substructure. 

Surprisingly, imino carbon shifts of strained 2 7/-azirines [431] do not differ substantially from those of open-chain imines (Section 4.9.3). On the other hand, the alkene carbon shifts of l-azabicyclo[2.2.2]oct-2-ene do not reflect the polarization characteristic of other enamines (Section 4.9.2) (+ )-M electron donation of the bridgehead enamino nitrogen cannot take place since the lone-pair orbital and the p orbitals of the it bond are 

Coumarin 7-Hydroxycoumarin 6-Hydroxycouma.rin 6,7-Dihydroxycoumarin Cyclic Enol Ethers and Related Compounds [419, 420] 

2-Phenyl- 2/f-azirine 3-Phenyl-2 H-azirine 2,3,4,5,6,7,8 a-Octa-hydroquinoline  

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